The present invention relates to a method for drying laundry and a machine able to dry laundry implementing such a method.
As known, some types of household appliances have the capability of drying the laundry after washing; typically, these appliances are the so-called wash-and-drying machines performing both the washing and the subsequent drying of the laundry, and laundry drying machines, which are only provided for drying the laundry.
These machines have a drum for containing the laundry to be dried, which is made to rotate for a preset time at a determined speed, while warm air is introduced into the tub wherein the drum is rotating; the drum rotation speed during drying is relatively low, typically about 55 revolutions per minute (r.p.m.); the drying time is generally set manually by means of a dedicated timing device, according to the user's specific needs (i.e., substantially as a function of the degree of drying to be reached and the type of clothes to be dried).
In order to dry the laundry, a certain “steady-state” temperature should be maintained within the tub wherein the drum is rotating, usually about 130° C.; to this purpose, the machine is provided with a suitable air circulation and heating system, comprising a blower, one or more electric resistances and condensing means for the damp air sucked from the tub.
Quite schematically, the damp air in the tub is sucked through the blower intake branch, along which the condensing means are also located; one or more electric resistances provided along the blower delivery side heat the dehumidified air before reintroducing it into the tub.
More conventional systems have only one air heating resistance, whose power is substantially sized for maintaining the steady-state temperature, of about 130° C. as an average.
Such an embodiment is shown schematically by way of example in FIG. 1.
In this figure, reference L and N indicate the phase and neutral wire of the 220 VAC mains supply and reference R indicate the above heating element or resistance, rated 1.000 W by way of example.
The heating element R is driven by a suitable electric switch C; in the instance of machines fitted with an electromechanical control system, this switch is a contact of a timer, whereas for machines fitted with an electronic control system, it may consist of the movable contact of a suitable relay.
Reference T indicates a thermostat of a known type, connected in series to the supply of the heating element R, which prevents reaching potential damaging temperatures within the machine tub; let assume that the upper threshold of intervention of the thermostat T is 140° C.
Upon starting the drying program, the switch C is made to closed and is maintained in this condition for the whole program, so as to enable the supply of the heating element R; in the event the threshold temperature is exceeded, the thermostat T cuts off the supply to the heating element R and activates it again, if required, should the actual temperature detected by the thermostat T decrease below the lower threshold of intervention of the latter.
The system illustrated in FIG. 1 represents a simple and cost-effective system, but is featured by a poor performance.
A first drawback related to the solution shown in FIG. 1 is represented by the fact that the control of the supply of the heating element R is assigned to an electromechanical thermostat, which typically has a high differential or hysteresis.
This problem can be cleared considering, for example, that the higher is the laundry load to be dried within the machine drum, the higher will be the volume increase of the laundry within the drum during the drying process, above all after the initial phase (i.e. with the clothes being still soaked with water and piled together); therefore, the space in the tub for the warm air decreases, so causing a temperature increase. As a result, at a certain time of the drying program, the air temperature within the tub will inevitably exceed the threshold of intervention of the thermostat T; as mentioned above, thermostats commonly employed to this purpose are featured by a high differential.
Upon exceeding the threshold temperature, here assumed to be 140° C. as mentioned, the thermostat T interrupts the supply to the heating element R, through the opening of one of its contacts, and the temperature in the tub will gradually decrease. Before the electric contact of the thermostat T closes again for a new supply to the heating element R to continue the drying program, the air temperature within the tub decreases to about 110° C., due to the cited differential.
Even if the decrease of the temperature in the tub is relatively fast, it is clear how the system described with reference to FIG. 1 is not very efficient, due to its waste of time and thermal energy, above all considering that during a drying program, particularly in the instance of a high laundry load, the electric contact of the thermostat T will forcedly be subject to a plurality of opening/closing cycles.
Another drawback related to the solution of FIG. 1 is represented by the fact that the heating element R is specifically sized for maintaining the rated drying temperature, i.e. the cited 130° C.
However, due to this sizing, the initial phase of the drying, during which the rise to the rated temperature is realized, requires a relatively long time, i.e. representing a further poor element of the system.
Washing machines and/or drying machines fitted with two electric resistances or heating elements for the air heating have been suggested, in order to reduce the above drawbacks, as highlighted in FIG. 2.
Both heating elements, indicated with R1 and R2, are driven by relevant electric switches C1 and C2, of the previously mentioned type, and provide respective thermostats T1 and T2 connected in series to the supply, which are calibrated for operating at different temperatures. In the illustrated example, the heating element R1 has a 1.000 W power, suitable for maintaining the rated drying temperature, whereas the heating element R2 has a 500 W power; the thermostat T1 is calibrated for threshold of intervention of about 140° C. whereas the thermostat T2 is calibrated for a threshold of intervention of about 125° C.
Upon starting a drying program, switches C1 and C2 are made to close and maintained in that condition for the whole program, so enabling the supply to the heating elements R1 and R2.
Thus, the sum of the individual heating powers of the two heating elements R1 and R2 allows a fast achievement of the rated drying temperature.
As said, the thermostat T2 is calibrated for a lower threshold of intervention compared to the threshold of thermostat T1 provided for controlling the rated temperature; it should also be noticed that the differential of the two thermostats T1 and T2 remains substantially always the same.
This means that, upon reaching 125° C., the electric contact of the thermostat T2 will open and inhibit the power supply to the heating element R2, whereas the heating element R1 is still supplied for air heating as required to maintain the rated temperature.
Should the temperature inside the tub exceed the 140° C. safety threshold, the thermostat T1 would stop supplying the heating element R1 through the opening of its own contact, so that the temperature in the tub will gradually decrease.
Also in this case, before the electric contact of the thermostat T1 can close again for a new power supply to the heating element R1, as required to carry on the drying program, due to the above thermostats differential the air temperature inside the tub has to drop down to about 110° C.
It should be noticed, here, that in this circumstance the contact of the thermostat T2 remains in the open condition, since its lower switching temperature (95° C.) will always be lower compared to the lower switching temperature of the thermostat T1 (110° C.).
Therefore, as it can be noticed, performance of the drying program can be partially improved by the solution illustrated in FIG. 2, i.e. reducing the rise time to the rated temperature.
However, this is an expensive solution, in as much as it presumes the use of two heating elements, two control contacts and two thermostats.
As an alternative to the solution described above, it should be noticed that the inhibition of the supply to the heating element R2 might be obtained directly through the contact C2 instead of a thermostat; to this purpose, the machine control system (either electromechanical or electronic) will control the opening of the contact C2 after a fixed time since starting the drying program (e.g. 10 minutes), so that during the further program development air heating is ensured by the heating element R1 alone.
However, also this solution is not a very practical one, since it requires the use of two heating elements, two supply contacts and one thermostat.
Apart from the type of control employed for the heating element R2, the above solution would not prevent the problem previously mentioned of the cyclicity of intervention of the thermostat T1, should the safety temperature be exceeded.